1. Field of the Invention
The present invention relates to a laser apparatus for use in the fields of medical services (diagnoses and treatments), physicochemistry, and industries.
2. Description of the Related Art
A gas laser apparatus is generally used as a conventional laser apparatus capable of multi-wavelength simultaneous oscillation. Laser apparatuses capable of outputting white light are announced in B. . Aran, B. K. E a 3 bI e B (C(25)791, 1976), FU YUN peng (Laser, 7-12, 10, 1980), or B. B. B a H e p, p (K a B a H T. e K T p, (10)672, 1983).
Also, as a continuous variable wavelength laser apparatus, an OPO (Optical Parametric Oscillator) laser which uses wavelength conversion using a nonlinear optical crystal is considered as a promising easy-to-maintain, solid-state laser because this laser has a wide variable wavelength range. J. A. Gioordmaine et al. (Phys. Rev. Lett., (14)973, 1965) first reported oscillation of an OPO laser using LiNbO.sub.3 (lithium niobate). However, this oscillation was not put into practical use because the damage threshold of the crystal was too low. In 1985, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences announced BBO: .beta.-BaB.sub.2 O.sub.4 (beta barium borate) as a nonlinear optical crystal with a high damage threshold (Y. X. Fan, R. C. Eckardt, R. L. Byer, C. Chen and A. D. Jiang). In 1986, the first OPO oscillation using the BBO crystal was confirmed (Optical Society of America, Washington, D.C., 1986). Since then, it has become possible to continuously output wavelengths over a broad range of 410 to 2,600 nm by using a single BBO crystal, and this method has been put into practical use.
Conventionally, as a resonator for use in an OPO laser apparatus, an apparatus which reciprocates a laser beam by using parallel plate mirrors is known. For example, Yunping Wang et al. announced an apparatus in Appl. Phys. Lett. 58(14), Apr. 8, 1991, pp. 1461-1463, and Tang C. L. et al. announced an apparatus in Proc. IEEE 80 pp. 365-374, 1992.
In these apparatuses, dielectric multilayered films are formed on the surfaces of parallel plate mirrors constituting an oscillator. Accordingly, these dielectric films are burned and damaged while optical amplification is repetitively performed.
To solve this problem, Sheng Wu (Division of Chemistry, California Institute of Technology) and Zhao Yang Sun (CASIX CO., China) announced an apparatus using a ring resonator capable of constituting an oscillator without using dielectric films in CLEO, 96(1996), "An OPO ring cavity with no coating". In this apparatus, an input/output mirror is arranged at an angle of 45.degree. to an excitation optical axis, and two BBO nonlinear optical crystals are arranged in an oscillation circuit.
Of the conventional apparatuses described above, however, a gas laser apparatus as one conventional multi-wavelength laser apparatus is large-scaled and lacking stability. A conventional OPO laser apparatus can continuously vary the wavelength by using a single nonlinear optical crystal. On the other hand, the apparatus cannot simultaneously output laser beams with a plurality of wavelengths, and this has limited the applications of the apparatus.
In a continuous variable wavelength laser apparatus, an input/output mirror is arranged at an angle of 45.degree. to an excitation optical axis as described above. Consequently, both the excitation efficiency and the oscillation efficiency are low, and this increases the threshold value of oscillation.
Also, when two nonlinear optical crystals are arranged in a ring-like oscillation circuit, a device for pivoting the nonlinear optical crystals to change the wavelength of an oscillating laser is complicated. This makes the apparatus difficult to put into practical use.